Set function to nil after running to allow GC

Backport upstream fix to syncext package to allow functions and their
transitive allocations to be GC'd after the function is invoked.

private/pkg/syncext/syncext.go

@@ -38,6 +38,7 @@ func OnceValue[T any](f func() T) func() T {
 			}
 		}()
 		result = f()
+		f = nil
 		valid = true
 	}
 	return func() T {
@@ -71,6 +72,7 @@ func OnceValues[T1, T2 any](f func() (T1, T2)) func() (T1, T2) {
 			}
 		}()
 		r1, r2 = f()
+		f = nil
 		valid = true
 	}
 	return func() (T1, T2) {
@@ -105,6 +107,7 @@ func OnceValues3[T1, T2, T3 any](f func() (T1, T2, T3)) func() (T1, T2, T3) {
 			}
 		}()
 		r1, r2, r3 = f()
+		f = nil
 		valid = true
 	}
 	return func() (T1, T2, T3) {